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1. In direct heating the combustion products
are mixed with the drying air meaning that they come in contact with the paddy.
2. In western countries this is only allowed for products used to feed animals.
3. In SE Asia direct fired heaters are not considered as problem because the flue gases will only pollute the rice hull, which is not considered a problem since the hull is removed during the milling process.
4. Indirect heating, on the other hand, involves a heat exchanger for heating up the drying air.
5. It adds cost and decreases the total fuel efficiency of the dryer.